The present invention relates to the control of internal combustion engines. More specifically, the present invention relates to a method and apparatus to control exhaust emissions in a variable displacement internal combustion engine.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
Variable displacement internal combustion engines (ICEs) provide for improved fuel economy and torque on demand by operating on the principal of cylinder deactivation. During operating conditions that require a relatively high output torque, every cylinder of a variable displacement ICE is supplied with fuel, air and spark to provide torque for the ICE. During operating conditions at low speed, low load and/or other inefficient conditions for a fully displaced ICE, cylinders may be deactivated to improve fuel economy for the variable displacement ICE and vehicle. For example, in the operation of a vehicle equipped with an eight-cylinder variable displacement ICE, fuel economy will be improved if the ICE is operated with only four cylinders during relatively low torque operating conditions by reducing throttling losses.
Throttling losses, also known as pumping losses, are the extra work that an ICE must perform to pump air from the relatively low pressure of an intake manifold through the ICE and out to the atmosphere. The cylinders that are deactivated will not allow air flow through their intake and exhaust valves, reducing pumping losses by forcing the ICE to operate at a higher intake manifold pressure. Since the deactivated cylinders do not allow air to flow, additional losses are avoided by operating the deactivated cylinders as xe2x80x9cair springsxe2x80x9d due to the compression and decompression of the air in each deactivated cylinder.
During the operation of the ICE, if the ICE is in a condition where it is inefficient to operate with the full complement of cylinders, a controller will deactivate the mechanisms operating the valves for selected cylinders and also shut off fuel to the cylinders. During a high torque demand condition for the ICE, the cylinders will be reactivated by operating the valves and supplying fuel to the cylinders.
Air-fuel ratios may also be adjusted to improve fuel economy in a vehicle. An air-fuel mixture is represented by a ratio called the equivalence ratio which is represented by the symbol xcex. The equivalence ratio is defined by the following equation:   λ  =            (              air        /        fuel            )              (                        air          /          fuel                ⁢                  xe2x80x83                ⁢        stoichiometry            )      
A relatively low air/fuel ratio below 14.7 (xcex less than 1) is characterized as a rich mixture, and an air fuel ratio above 14.7 (xcex greater than 1) can be characterized as a lean mixture. Traditional vehicle engines are operated at stoichiometry since most regulated gases are reduced at stoichiometry. If vehicle engines are operated as a lean mixture, such as found in direct injection engines having lean stratified operations, fuel economy may be improved, but the production of NOx compounds is increased due to the increase in oxygen in the air-fuel mixture. The additional NOx compounds generated by a lean burning engine may not be sufficiently reduced by a traditional three-way catalyst to meet present regulations.
The present invention is a method and apparatus for the control of NOx emissions in a variable displacement engine. In the preferred embodiment of the present invention, an eight-cylinder internal overhead valve (OHV) internal combustion engine (ICE) may be operated as a four-cylinder engine by deactivating four cylinders, but any overhead cam (OHC) ICE equipped with cylinder deactivation is within the scope of the present invention. The cylinders in the preferred embodiment are deactivated by an electro-hydraulic deactivation system using engine oil pressure, controlled by solenoids, to pressurize the locking pins of special engine valve lifters (as used in an OHV engine). With pressure applied, the locking pins allow the lifter to act as a lost motion device to prevent exhaust and intake valve activation.
Under lean operating conditions such as found in direct injection engines, or engines operating under lean stratified conditions, a NOx trap is used to capture NOx emission from the exhaust gas flow. Periodically (once the NOx trap is saturated or at some predetermined threshold), the air-fuel mixture is adjusted to a rich mixture to regenerate the NOx trap and chemically reduce the NOx trapped in the NOx trap. Hydrocarbons and carbon monoxide generated by the rich air-fuel mixture will react with the released NOx to reduce the NOx into nitrogen N2 and oxygen O2. The frequency of NOx trap regeneration is calibrated as a function of the capacity of the NOx trap. A drawback to this technique is that some or all of the fuel economy gained through lean operation is lost in the regeneration mode. Careful timing of the enrichment of the air-fuel mixture can help minimize fuel losses but they are still relatively significant.
The deactivation of cylinders in the present invention significantly reduces fuel consumption under lean and rich operating conditions. By switching from lean operation, when the engine is in a fully displaced mode, to a partially displaced mode, the ICE can operate efficiently at homogeneous or rich fuel-air ratios and still provide improved fuel economy over a homogeneous fully displaced ICE operation. The overall effect is that fuel economy improvements generated by lean operation in a fully displaced operating mode will not be canceled out by rich operation in a partially displaced mode when the NOx trap is regenerated. Accordingly, variable displacement engines may be run in a partially displaced mode with an air-fuel ratio richer than stoichiometric to regenerate the NOx trap without the fuel economy penalties generated by running a rich air-fuel mixture during fully displaced operation.